Dust cover superior in transparency for photomask reticle use and process for producing the same

ABSTRACT

In a dust cover for photomask reticle purposes which consists essentially of a supporting frame and a thin film bonded to a surface of the frame, the dust cover is improved in transparency by constructing at least the outermost layers of the thin film with thin films of a fluoropolymer which exhibits an average transmittance of at least 90% for rays of wavelengths from 240 to 290 nm and an average transmittance of at least 93.5% for rays of wavelengths from 290 to 500 nm, when having a thickness of 10 μm, and has a refractive index of up to 1.42.

TECHNICAL FIELD

The present invention relates to a dust cover superior in transparencyfor photomask reticle use. More particularly, the invention relates tosuch a dust cover comprising a film having at least thin outermostlayers of a specific fluoropolymer, the film being bonded to asupporting frame.

BACKGROUND ARTS

With the densification of integrated semiconductor circuits in recentyears, printed wires in the circuits have been extremely fined to widthsof from 1 to 3 μm and the lithographic technique of forming images ofsuch wires on wafers are being changed from the projection system to thestepper system.

When dust is present on a photomask reticle in the lithographic process,images of dust particles will also be formed, resulting in shortcircuits and defects in the produced circuit, and this will lower theyield of the LSI. Specially in the stepper system, several images on areticle are successively projected on a reduced scale to a wafer. Henceone dust particle on a reticle may make defective all the resultingLSI's. Thus it is getting very important to reduce the dust to zero.

Thereupon, a method for preventing dust adhesion onto photomask reticles(hereinafter abbreviated occasionally as "mask") has been proposed (U.S.Pat. No. 4,131,363). The method comprises arrangement of a transparentfilm or films over one or both sides of the mask with a space leftbetween the film and the mask surface. According to this method, sincedust adheres only to the transparent film, defects due to dust can beprevented by focussing projecting rays on the image prepared on the maskand bringing the dust adhering onto the film out of focus, so that theimage of the dust is not formed on the wafer.

The dust cover, applied to the mask, is set in an exposer. The film,which is the principal part of the dust cover, is disposed in theoptical path of exposure light.

Consequently, the film needs to transmit light without causing thedistortion or disorder of image, have a uniform thickness, and be freeof foreign matter, fault, and internal strain.

The transparency of the film to the exposure light is also important.That is, when the transparency is low, the exposure needs to be thelonger and therefore the throughput will be the less. In the productionof LSI, it is very important to raise the throughput since LSI's in manycases are produced in extremely large volumes. Specially in the steppersystem, the exposure in some cases is repeated hundreds of times per onewafer and the improved transparency of the dust cover film hencecontributes greatly to the rise in the throughput.

For the film of the dust cover, nitrocellulose is in use. Althoughnitrocellulose is used because of the high film strength thereof and thecapability thereof to give very uniform films, the light transmittanceof these films is about 92% at wavelengths of from 350 to 450 nm whichare of the exposure light used today. Thus nitrocellulose films as suchare insufficient in light transmittance for use as the film of the dustcover. In consequence, a method has been proposed, in which theinterference of two light waves reflected from the front and rearsurfaces of the film is utilized, that is, the film thickness is chosenso that these reflected waves will cancel each other (U.S. Pat. No.4,378,953). According to this method, the light transmittance rises to99% while the film thickness becomes 0.865 μm, being extremely thin.Hence this method has the drawback of requiring the exercise ofsufficient care in handling such a dust cover. On the other hand, theincrease in the film thickness will extremely narrow the tolerance ofthe thickness in order to achieve a light transmittance of 98% or more,thus making the film production very difficult. In addition, if the filmthickness is increased to 6 μm or more, the light transmittance thereofwill be lowered on account of minute dimensional roughness of the filmsurface.

Another method is to prevent the reflection with an inorganic compoundlayer formed by vapor deposition or the like on both sides of anitrocellulose film. This method is an application of the technique usedfor optical lens and eyeglasses. According to this method, thereflection of light of any possible wavelength as desired can beprevented by choosing the thickness of the anti-reflection layer and alight transmittance of 98% or more can be attained for a relativelythick film.

In order to form such films, however, an expensive apparatus such as avacuum deposition arrangement is necessary and additionally the vapordeposition needs to be carried out at a low temperature so as not toimpair the cellulose film. In the low temperature vapor deposition, theinorganic compound to coat is not densely deposited and the refractiveindex of the deposit layer varies with the temperature condition. Hencethe deposit layer may not exhibit the intended anti-reflection function.Moreover, the deposited material, depending on the nature thereof, maybe oxidized with air, resulting in a change in the refractive index.Accordingly, the function of the resulting dust covers is not definite.

On the other hand, as LSI's are more densified, there is a growingdemand for printed wire widths of submicron order. To meet such ademand, rays of wavelengths from 350 to 450 nm used today for theexposure do not have enough resolving power and rays of shorterwavelengths, i.e. ultraviolet rays of wavelengths from 240 to 290 nmneed to be used. However, nitrocellulose films show rapid decrease inthe light transmittance with decrease in wavelength from 300 nm and aredegraded by far ultraviolet rays to such an extent that it can be nolonger used. Therefore a novel dust cover is looked for.

DISCLOSURE OF THE INVENTION

According to the present invention, there is provided a dust coversuperior in transparency for photomask reticle purposes, which consistsessentially of a supporting frame and a thin film bonded to edge areasthereof, said thin film consisting of either a single layer of afluoropolymer which exhibits an average transmittance of at least 90%for rays of wavelengths from 240 to 290 nm and an average transmittanceof at least 93.5% for rays of wavelengths from 290 to 500 nm, whenhaving a thickness of 10 μm, and has refractive index of up to 1.42 orthree or more multilayers both the outermost layers of which are formedof the above-defined fluoropolymer. The anti-reflection dust coverconsisting essentially of a layer formed of the above fluoropolymer anda supporting frame and the anti-reflection dust cover having a thin filmof the above fluoropolymer as the core layer are superior intransparency to far ultraviolet rays of wavelengths from 240 to 290 nm.Dust covers of constant quality, without being varied in theanti-reflection function, having improved transparency to rays ofwavelengths from 350 to 450 nm used today for the exposure can also beformed by covering both sides of thin nitro-cellulose films with thefluoropolymer. The present invention also provides a process forproducing a dust cover superior in transparency for photomask reticleuse, characterized in that each layer of the film, which is theprincipal portion of the dust cover, is formed from a solution of theprescribed polymer in a solvent by a coating method, preferably thespinner coating method. According to the process of the invention, eachlayer can be formed with ease and high reproducibility.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an enlarged sectional view showing an example of the filmwhich is an important element of the present invention.

FIG. 2 is an enlarged sectional view showing another example of the filmwhich is an important element of the invention.

FIG. 3 is a cross-sectional view of the entire body of the dust cover.

FIG. 4 is a cross-sectional view showing an embodiment of using the dustcover of the invention.

FIG. 5 shows the relation between wavelength and light transmittancemeasured on the dust covers of Example 1 and of Comparative Example 1.

FIG. 6 shows the relation between wavelength and light transmittancemeasured on the dust covers of Example 3 and of Comparative Example 2.

FIG. 7 shows the relation between wavelength and light transmittancemeasured on the dust cover of Example 10.

FIG. 8 shows the relation between wavelength and light transmittancemeasured on the dust cover of Comparative Example 3.

FIG. 9 shows the relation between wavelength and light transmittancemeasured on the dust cover of Example 11. In FIGS. 1 to 4, numeralsdenote the following:

1: core film, 2: intermediate layers, 3: outmost layers, 4: film, 5:frame, 6: bond layer, 7: bond layer, 8: protective film, 9: mask, 10:image.

BEST MODE FOR CARRYING OUT THE INVENTION

Since the transparency of a film to light is lowered with theabsorption, scattering, and reflection of the light by the film, a dustcover superior in transparency can be obtained by eliminating completelyor partly these phenomena.

The ability of a film to absorb light is a property of the film itself.Hence, it is necessary to choose a polymer superior in transparency inorder to use as a film for the dust cover. Films in use today for thedust covers are those of nitrocellulose. Nitrocellulose, althoughsuperior in transparency to rays of wavelengths from 350 to 450 nm,absorbs markedly far ultraviolet rays of wavelengths up to 300 nm andtherefore cannot be used for the dust cover in these wavelength region.In contrast to this, the fluoropolymer used in the present inventionscarcely absorbs far ultraviolet rays of wavelengths from 240 to 290 nm,being superior in transparency to these rays.

The scattering of light by a polymer is attributable to dust containedin the polymer and to crystals of the polymer. The dust in polymers canbe removed by methods such as through purification. The problem ofpolymer crystals does not arise when the polymer is amorphous. Henceamorphous polymers are much preferred though a crystalline polymer maybe used satisfactorily if it can be brought into an amorphous state byquenching or some other method.

On the other hand, the reflection of light by a polymer is attributableto the refractive index of the polymer; the higher the refractive indexthe greater the reflectivity. Since the refractive index ofnitrocellulose is about 1.5, the reflectivity of films thereof will beabout 8% and hence the average light transmittance of these films willbe up to 92% in the wavelength region of from 350 to 450 nm wherein theabsorption or the scattering is not caused by these films. In contrast,the fluoropolymer used in the present invention, having a low refractiveindex of up to 1.42, will show a low reflectivity of up to about 6% andan average light transmittance of about 94% or higher.

As described above, the dust cover of the present invention comprising afilm of a specific fluoropolymer as the principal portion shows superortransparency to not only far ultraviolet rays of wavelengths from 240 to290 nm but also rays, used today for the exposure, of wavelengths from350 to 450 nm.

Any fluoropolymer may be used in the present invention that has anaverage light transmittance of at least 90% for rays of wavelengths from240 to 290 nm and an average light transmittance of at least 93.5% forrays of wavelengths from 290 to 500 nm, when having a thickness of 10μm, and has a refractive index of up to 1.42. Examples of suchfluoropolymers are a tetrafluoroethylene (TFE) - hexafluoropropylene(HFP) copolymer, vinylidene fluoride homopolymer, vinylidene fluoride(VdF)-TFE copolymer, TFE-perfluoroalkyl vinyl ether copolymer, andVdF-ethylene copolymer. Preferred fluoropolymers that can be dissolvedin solvents and formed into uniform films by a coating method,particularly by the spinner coating method, are a vinylidene fluoridehomopolymer, vinylidene fluoride-tetrafluoroethylene copolymer whereinthe content of vinylidene fluoride is at least 50 mole %, andtetrafluoroethylene (TFE)-hexafluoropropylene (HFP) - vinylidenefluoride (VdF) terpolymer wherein the content of tetrafluoroethylene isfrom 35 to 65 mole % and the molar ratio of HFP to VdF is from 1:1 to1:2.3.

A dust cover further superior in transparency, when necessary, can beobtained by making the film, which is the principal portion of thecover, into a multilayer film having layers of the fluoropolymer, usedin the present invention, on the outermost sides. That is, since thefluoropolymer, as stated above, is superior in transparency to rays overa wide wavelength range from far ultraviolet rays to ordinaryultraviolet rays and has a low refractive index, the reflection can beprevented by disposing films of the fluoropolymer as the outermostlayers and canceling the reflected waves with each other; thus theresulting multilayer film will show better transparency. This is basedfundamentally on the same theory as in the case of the above-mentionedanti-reflection dust cover formed by vapor deposition of an inorganiccompound on a nitrocellulose film. According to the present invention,however, the anti-reflection layers are formed of the polymer, therebyan expensive apparatus such as a vacuum deposition arrangement being notrequired, and a more important matter is that all the layers are formedof polymers and therefore the anti-reflecting function scarcely variesand dust covers having constant functions are obtainable. This is agreat advantage of the invention.

As shown in FIG. 1, an example of the structure of the film which is theprincipal portion of such an anti-reflection dust cover is formed bydisposing the outermost layers 3 of a polymer on both sides of a corefilm 1, the polymer film having a reflactive index lower than that ofthe film 1 and an optical thickness (refractive index×thickness)corresponding to 1/4 of the wavelength of the light to be prevented fromreflection. Since the accuracy of this thickness much affects theanti-reflecting function, the satisfactory anti-reflecting function willnot be afforded unless the thickness is controlled to suppress thedeviation from the prescribed thickness within 10%. When the light to beprevented from reflection is a usual ultraviolet ray having a wavelengthof 350 to 450 nm, a nitrocellulose film of 0.5 to 15 μm in thickness isused as the core film 1 and the above-mentioned fluoropolymer is usedfor the films 3 to be disposed separately on both sides thereof. In thiscase, the conditions for the complete prevention of the reflection are:

    n.sub.3 =√n.sub.1

    n.sub.3 d.sub.3 =1/4λ(2N+1)

wherein n₁ is the refractive index of the core film 1, n₃ is therefractive index of the film 3 to be disposed on both sides of the film1, d₃ is the thickness of the films 3, λ is the wavelength of the lightto be prevented from reflection, and N is 0.1, or 2. Since therefractive index of nitrocellulose is 1.5, no substance satisfying thecondition of n=√1.5 exists and hence the complete prevention ofreflection is impossible. However, the fluoropolymer has considerableanti-reflection effect since the refractive index thereof as low as 1.42or less and the average light transmittance of the whole film will be atleast about 96%, being improved by about 4% or more over the film formedof nitrocellulose alone. The value of N in the above equation ispreferably 0. Although the anti-reflecting effect is also produced whenN is 1 or 2, the wavelength range of rays on which the effect isobserved will be restricted and the tolerance of the thickness willnarrower, thus making the production difficult.

Another example of the film, as shown in FIG. 2, can be formed bydisposing intermediate layers 2 of a polymer on both sides of a corefilm 1, the polymer having a refractive index higher than that of thecore film 1, and disposing the outermost layers 3 of another polymer onthe outsides of the intermediate layers 2, the latter polymer having arefractive index equal or lower than that of the core film 1, and theoptical thickness of each of the intermediate and outermost layerscorresponding to 1/4 of the wavelength of the light to be prevented fromreflection. That is, when the light to be prevented from reflection is afar ultraviolet ray having a wavelength of 240 to 290 nm, thefluoropolymer is used for the core film 1, a transparent polymer havinga refractive index of at least 1.45 is used for the intermediate layers2 to be disposed on both sides of the core film 1, and the fluoropolymeris used for the outermost layers 3. When the light to be prevented fromreflection is a usual ultraviolet ray having a wavelength of 350 to 450nm, the above-mentioned three-layer film can of course be used while afive-layer film may also be formed by using nitrocellulose for the corefilm 1, a transparent polymer having a refractive index of at least 1.57for the intermediate layers 2 on both sides of the core film 1, and thefluoropolymer for the outermost layers 3 on the outer sides of theintermediate layers 2.

The conditions for preventing the reflection with the film of such astructure are:

    n.sub.2 /n.sub.3 =√n.sub.1

    n.sub.2 d.sub.2 =n.sub.n d.sub.3 =1/4λ(2N+1)

wherein n₁ is the refractive index of the core film 1, n₂ is therefractive index of the intermediate layers 2, d₂ is the thicknessthereof, n₃ is the refractive index of the outermost layers 3, d₃ is thethickness thereof, λ is the wavelength of the light to be prevented fromreflection, and N is 0, 1, or 2. In practice, the multilayer film showsa superior anti-reflecting function without complete satisfaction of theabove equations, provided that the refractive index of each layer ischosen to approximate to the proper value. The value of N in the aboveequation is preferably 0. Although the anti-reflecting effect is alsoproduced when N is 1 or 2, the wavelength range of rays on which theeffect is observed will be restricted and the tolerance of the thicknesswill be narrower, thus making the production difficult.

For instance, when the core film 1 is formed of the fluoropolymer havinga refractive index of 1.37, the intermediate layers 2 are formed of apolymer having a refractive index of 1.47, and the outermost layers 3are formed of the same fluoropolymer as that of the core film 1, theaverage transmittance of the whole film will be 96.5% at 280 nm, beingimproved by 2% or more over that of the single film. When the refractiveindex of the intermediate layers 2 is lower than 1.45, or when therefractive index of the outermost layers is higher than that of the corefilm 1, the anti-reflecting effect will be practically lost or on thecontrary, the reflection may become increased sometimes. Hence greatcare must be taken in selecting the refractive index of each layer.

Further, for instance, when the core film 1 is formed of nitrocellulose,the intermediate layers 2 are formed of a polymer having a refractiveindex of 1.59, and the outermost layers 3 are formed of thefluoropolymer having a refractive index of 1.37, the whole film willshow a transmittance of 99.5% for the ray of wavelength 436 nm which isin use today as a light source for exposure in the stepper system.

When the core portion is formed of the fluoropolymer, any polymer, whichhas a refractive index of at least 1.45 and well transmits farultraviolet rays of wavelengths from 240 to 290 nm, may be used for theintermediate layers 2. Examples of such polymers are ethylcellulose andacetylcellulose and the like. When the core film 1 is formed ofnitrocellulose, there may be used, for the intermediate layers 2, apolymer which has a refractive index of at least 1.57 and well transmitsusual ultraviolet rays of wavelengths from 350 to 450 nm. Such polymersinclude, for example, polystyrene, polysulfone, polyethersulfone,polyphenylene ether, polycarbonate, and aromatic polyester.

As the thickness of the films used in the present invention, a film ofthe fluoropolymer, when used singly or as the core film, is desirablyfrom 1 to 15 μm, and a film of nitrocellulose, when used as the corefilm, is desirably from 0.5 to 15 μm. When the thickness of each film isless than the above value, the film strength will be too low for easyhandling, and when the thickness exceeds the above value, a uniform filmof good precision is hard to form, such thickness being of no advantage.

The deviation in the position of the mask image projected, viz, thefocus shift with an increase in the film thickness is as small as about0.1 to 0.2 μm in the stepper system even when the film thickness is 15μm. Thus the film thickness has little effect on the image formation.

The structure of the portion other than the film portion, of the dustcover is as shown in FIG. 3, being fundamentaly the same as usedconventionally. In the dust cover structure, the film 4 is fixed througha bond layer 6 on a supporting frame 5 without forming any wrinkle orsagging. On the other side of the supporting frame, a sticky layer 7 isformed and overlaid with a protective film 8. The supporting frame 5 isusually made of an aluminum alloy or a plastic, and has a diameter of 1to 6 inches or a square of about 1 to 6 inches and a height of about 2to 10 mm

The dust cover is contained in a clean case until use to be protectedfrom dust. When using, the protective film 8 shown in FIG. 3 is peeledoff. The dust cover of the present invention is set to cover an image 10on a mask 9. The mask and the dust cover is stored also in the formshown in FIG. 4, thereby preventing the dust adhesion to the image areaand making the mask cleaning unnecessary before reusing.

Methods applicable to the production of the films in various embodimentsdescribed above include solution cast methods such as the spinnercoating method, the coater method using a knife coater, rod coater, orthe like, and the dip coating method, when the polymer to use is solublein a solvent; and melt extrusion methods such as the T-die extrusionmethod and the inflation extrusion method, when the polymer is athermoplastic resin. Of these methods, the spinner coating method insolution cast methods is most recommendable since the accuracy of filmthickness and the uniformity of film thickness are very important forthe film used in the present invention and the production of the filmfree of dust and faults is essential.

The spinner coating method comprises dissolving a polymer in a solvent,dropping this solution on a flat and smooth substrate or on an alreadyformed film, and rotating the substrate or the film at a suitable numberof revolutions, thereby forming a film of a desired thickness. Accordingto this method, dust and the like can be removed completely by filteringthe solution and the development of faults and the like can be reducedto zero by using a cleaned silicon wafer or a smooth glass plate as thesubstrate and additionally a desired thickness of the film can beobtained by proper selection of the solution concentration and thenumber of revolutions for coating. Moreover the uniformity of filmthickness is satisfactory.

When the film is formed only of a layer of the fluoropolymer, the dustcover is made in such way that a film of the polymer is formed on asmooth substrate such as a silicon wafer by the spinner coating methodor the like, the surface of an adhesive previously applied on asupporting frame is brought into contact with a surface of the film tobond the film to the frame, and the assembly is dipped in water toseparate the film from the substrate. When the film is of a multilayertype, the dust cover is made in such way that the core film is formed onthe substrate, and after the intermediate layer has been formed on oneside of the core film or without forming the intermediate layer, theoutermost layer is formed thereon, the supporting frame is bondedthrough an adhesive to a surface of the resulting film, the substrate isseparated from the film, and then the other side of the core film isoverlaid successively with or without the intermediate layer, and withthe outermost layer, in the same manner as above.

The present invention is illustrated in more detail with reference tothe following examples.

In the following examples and comparative examples, the film thicknesswas determined in the following manner:

When the film was relatively thin and interference waves were observedin the measurement with a spectrophotometer (UV-240, supplied by ShimazuCo., Ltd.), the thickness of the film was calculated according to thefollowing equation:

    d=Kλ.sub.1 λ.sub.K /2n(λ.sub.1 -λ.sub.K)

d: Film thickness (nm)

λ₁ : Peak wavelength (nm) of interference waves in the vicinity ofD-line (587.6 nm).

λ_(K) : Wavelength (nm) of K-th peak from λ₁

n: Refractive index of film

When the film was relatively thick and the interference wave was notobserved, the film thickness was measured with a Schopper type ofthickness gage.

EXAMPLE 1

A 20 wt % solution of a tetrafluoroethylenevinylidene fluoride copolymer(supplied by Pennwalt Corp. under the tradename of Kynar 7201, TFEcontent about 10 mole %, refractive index 1.40) was prepared bydissolving the copolymer in methyl ethyl ketone.

A silicon wafer was set in a spin coater, the above solution was droppedon the wafer, and the wafer was rotated to form a thin coat on thewafer. This coat was dried by heating with an infrared lamp, and analuminum supporting frame one side of which had been coated with anepoxy adhesive was placed on the dried coat to bond itself thereto, andthe adhesive was cured.

This assembly was immersed in water and allowed to stand for 30 minutes.Then the wafer peeled off, and the film-supporting frame was withdrawnfrom the water and air-dried to make up a dust cover, which comprisedthe supporting frame and the film supported thereby.

The thickness of this film was about 14 μm, no interference wave due tothe film was observed in measurement with a spectrophotometer, and thetransmittance for light of wavelength 240 μm was 92.5% and the averagetransmittance for light of wavelengths from 350 to 450 nm was 94.5%.Thus the film showed a superior function over a wide range of from a farultraviolet region to an ultraviolet region (see FIG. 5).

COMPARATIVE EXAMPLE 1

A 10 wt % solution of nitrocellulose (HIG-20, supplied by Asahi KaseiKogyo Co., Ltd.) was prepared by dissolving it in n-butyl acetate.

Using this solution, a dust cover was made in the same manner as inExample 1.

The film of the obtained dust cover was 2.861 μm in thickness and showedinterference waves. The film was found to have a transmittance of 35%for light of wavelength 240 nm and a maximum transmittance of 98.2%,minimum transmittance of 84.8%, and average transmittance of 91.5%, inthe vicinity of a wavelength of 350 nm, and an average transmittance of92.0% for light of wavelengths from 350 to 450 nm (see FIG. 5).

EXAMPLE 2

A tetrafluoroethylene (TFE)-hexafluoropropylene (HFP)-vinylidenefluoride (VdF) mixture was polymerized at 85° C. by using afluorine-containing surfactant (C₈ F₁₇ COONH₄, supplied by 3M Co. underthe tradename of Fluorad FC-143) as emulsifier, ammonium persulfate asinitiator, and diethyl malonate as chain transfer agent. The TFE/HFP/VdFmolar ratio in the obtained terpolymer was calculated as 48.2/20.1/31.7from the balance of each monomer. The refractive index of thisterpolymer was 1.36. This terpolymer was dissolved inperfluoro-2-methyl-1-oxy-3-thiacyclohexane-3,3-dioxide ##STR1## toprepare a 3 wt % solution.

A dust cover was made by treating this solution in the same manner as inExample 1.

In spectrophotometry of the film of the obtained dust cover,interference waves were observed, and the film thickness was calculatedas 2.04 μm. The film was found to have a transmittance of 93.1% onaverage for light of wavelength 240 nm and an average transmittance of95% for light of wavelengths from 350 to 500 nm.

EXAMPLE 3 AND COMPARATIVE EXAMPLE 2

A 7 wt % solution of nitrocellulose (H-20, supplied by Asahi Kasei KogyoCo., Ltd.) was prepared by dissolving it in n-butyl acetate. Also an 0.6wt % solution of a TFE/HFP/VdF terpolymer (TFE-49.2 mole %, HFP=19.1mole %, VdF=31.7 mole %, N_(D) =1.35) was prepared by dissolving it inperfluoro-2-methyl-1-oxy-3-thiacyclohexane-3,3-dioxide.

A silicon wafer (125 mm φ) was set in a spin coater, and the abovenitrocellulose solution was dropped on the wafer to form a uniformnitrocellulose coat on the wafer. The coat was dried by heating with aninfrared lamp. On the dried coat was dropped the solution of theTFE/HFP/VdF terpolymer. The wafer was rotated at 700 rpm for 20 seconds.Then this coat was dried by heating with an infrared lamp. An aluminumring (supporting frame) of 100 mm φ (outer diameter) coated with anepoxy adhesive was placed on the terpolymer coat, and the adhesive wascured.

This assembly was immersed in water and allowed to stand for 30 minutes,and then the wafer was separated from the film. The film bonded to thealuminum ring was withdrawn from the water, air-dried, and set again inthe spin coater with itself positioned on the upper side. The solutionof the TFE/HFP/VdF terpolymer was dropped, and formed into a film byrotation under the same conditions as above. The coat was then dried tomake up the intended dust cover.

As is evident from FIG. 6, this anti-reflection film was found fromspectrophotometry to have transmittance of from 96.5 to 99.5% and anaverage transmittance of 98%, in the vicinity of a wavelength of 400 nm.On the other hand, in spectrophotometry of a dust cover having anitrocellulose film alone on which the TFE/HFP/VdF terpolymer was notapplied, large interference waves were observed and thus thetransmittance ranged from 84 to 99% and the average transmittance was92%.

EXAMPLE 4

A dust cover was made by repeating the procedure of Example 3 but usinga solution of a TFE/VdF copolymer (TFE content - 38.7 mole %, n_(D)=1.40) in hexamethylphosphoric acid triamide in place of the TFE/HFP/FdFterpolymer. From spectrophotometry of this dust cover, the transmittancein the wavelength region of from 350 to 450 nm was from 92.5 to 99.5%and the average transmittance was 96%, proving the anti-reflectioneffect.

EXAMPLES 5, 6, 7, 8 and 9

A 9 wt % solution of nitrocellulose (H-20, supplied by Asahi Kasei KogyoCo., Ltd.) was prepared by dissolving it in ethyl lactate. A polystyrene(GP-683, supplied by Asahi Kasei Kogyo Co., Ltd., n_(D) =1.59),polysulfone (P-3500, supplied by Nissan Chemicals Co., Ltd., n_(D)=1.63), polyethersulfone (P-200, supplied by ICI Co., Ltd., n_(D)=1.63), polycarbonate (1250, supplied by Teijin Co., Ltd., n_(D) =1.59),and polyphenylene ether (supplied by Asahi Kasei Kogyo Co. Ltd. underthe tradename of Xyron T-1045, n_(D) =1.59) were each dissolved in1,1,2,2-tetrachloroethane to prepare 0.75 wt % solutions. Further aTFE/HFP/VdF terpolymer (TFE - 44.2 mole %, HFP - 20.7 mole %, VdF - 35.1mole %, n_(D) =1.37) was dissolved inperfluoro-2-methyl-1-oxy-3-thiacyclohexane-3,3-dioxide to prepare a 0.6wt % solution.

Using a spin coater, a film of nitrocellulose, a film of one of thepolymers (polystyrene, polysulfone, polyethersulfone, polycarbonate, andpolyphenylene ether) as an intermediate layer, and a film of theTFE/HFP/VdF terpolymer were formed successively from the above solutionsover a silicon wafer. An aluminum ring was bonded to the resultinglaminate in the same manner as in Example 3, and the silicon wafer wasseparated from the laminate film in water. Further an intermediate layerand a layer of the TFE/HFP/VdF terpolymer were formed by using the spincoater. In this manner, dust covers having a nitrocellulose film as thecentral layer were made up. Found spectral characteristics of thesecovers were as shown in Table 1. Any of these covers showed a minimumlight transmittance of at least 98%, thus having sufficient performancecharacteristics as a dust cover for the stepper system.

                  TABLE 1                                                         ______________________________________                                        Example                                                                              Polymer for inter-                                                                         Light transmittance (%) (436 nm)                          No.    mediate layer                                                                              Minimum   Maximum Average                                 ______________________________________                                        5      Polystyrene  98.3      99.5    98.9                                    6      Polysulfone  99.5      99.5    99.5                                    7      Polyethersulfone                                                                           99.5      99.5    99.5                                    8      Polycarbonate                                                                              98.9      99.8    99.5                                    9      Polyphenylene                                                                              99.0      99.8    99.4                                           ether                                                                  ______________________________________                                    

EXAMPLE 10 AND COMPARATIVE EXAMPLE 3

A dust cover was made in the same manner as in Example 6 except that theconcentration of the nitrocellulose solution was changed to 14 wt %.Data of spectrophotometry of the formed film are shown in FIG. 7. Dataof the nitrocellulsoe film alone which is the core of the laminate filmare shown in FIG. 8. From FIG. 8, the thickness of the nitrocellulosefilm was calculated as 9.5 μm and the average transmittance in thevicinity of a wavelength of 436 nm was as low as 91.5% (ComparativeExample 3).

In contrast, while the thick of the core nitrocellulose film in Example10 was assumed to be similarly 9.5 μm, the transmittance at a wavelengthof 436 nm was as high as 99.5% in spite of such a heavy thickness of thefilm.

EXAMPLE 11

The TFE/VdF copolymer of Example 1 was dissolved in ethyl methyl ketoneto prepare a 20 wt % solution thereof. Acetylcellulose (refractive index1.48) was dissolved in an ethyl lactate/n-butyl acetate/ethanol(40/35/25 in weight ratio) mixed solvent to prepare an 0.6 wt % solutionthereof. Further the TFE/HFP/VdF terpolymer of Example 2 was dissolvedin perfluoro-2-methyl-1-oxy-3-thiacyclohexane-3,3-dioxide to prepare an0.6 wt % solution thereof.

Using these solutions, a multilayer dust cover having a film of theTFE/VdF copolymer as core layer, acetylcellulose films as intermediatelayers, and films of the TFE/HFP/VdF terpolymer as outmost layers wasmade in the same manner as in Example 5.

The thickness of the core TFE/VdF copolymer film was 2.25 μm and thetransmittance of the cover at a wavelength of 280 nm was on average97.5% (see FIG. 9).

We claim:
 1. A dust cover superior in transparency for photomask reticlepurposes, which consists essentially of a supporting frame and a thinfilm bonded to edge areas thereof, said thin film consisting of either asingle layer of a fluoropolymer which exhibits an average transmittanceat least 90% for rays of wavelengths from 240 to 290 nm and an averagetransmittance of at least 93.5% for rays of wavelengths from 290 to 500nm, when having a thickness of 10 μm, and has a refractive index of upto 1:42 or three or more multilayers both the outermost layers of whichare formed of the above-defined fluoropolymer.
 2. The dust cover ofclaim 1, wherein said thin film is of a single layer structure having athickness of 1 to 15 μm.
 3. The dust cover of claim 1, wherein said thinfilm has a three-layer structure and the core layer thereof is anitrocellulose film having a thickness of 0.5 to 15 μm.
 4. The dustcover of claim 3, wherein both the outermost layers has an opticalthickness corresponding to 1/4±10% of the wavelength of the light touse.
 5. The dust cover of claim 1, wherein said thin film has thefive-layer structure consisting of a core layer, two intermediate layersbonded separately onto both sides of the core layer, and two outermostlayers, the core layer is a thin film of fluoropolymer exhibiting anaverage transmittance of at least 90% for rays of wavelengths from 240to 290 nm and an average transmittance of at least 93.5% for rays ofwavelengths from 290 to 500 nm, when having a thickness of 10 μm, andhaving a refractive index of up to 1.42, the intermediate layers arethin films of a transparent polymer having a refractive index of atleast 1.45, and the outermost layers are thin films of the above-definedfluoropolymer having a refractive index not higher than that of the corefilm.
 6. The dust cover of claim 5, wherein the intermediate layers andthe outermost layers have each an optical thickness corresponding to1/4±10% of the wavelength of the light to use.
 7. The dust cover ofclaim 1, wherein said thin film has the five-layer structure consistingof a core layer, two intermediate layers bonded separately onto bothsides of the core layer, and two outermost layers, the core layer is anitrocellulose film having a thickness of 0.5 to 1.5 μm, and theintermediate layers are transparent polymer film having a refractiveindex of at least 1.57.
 8. The dust cover of claim 7, wherein theintermediate layers and the outermost layers have each an opticalthickness corresponding to 1/4±10% of the wavelength of the light touse.
 9. The dust cover of claim 2, wherein said fluoropolymer is ahomopolymer of vinylidene fluoride or a vinylidenefluoride-tetrafluoroethylene copolymer having a vinylidene fluoridecontent of at least 50 mole %.
 10. The dust cover of claim 1, whereinsaid fluoropolymer is a tetrafluoroethylene (TFE)-hexafluoropropylene(HFP)-vinylidene fluoride (VdF) terpolymer having a TEF content of 35 to65 mole % and an HFP: VdF molar ratio of from 1:1 to 1:2.3.
 11. The dustcover of claim 2, wherein said fluoropolymer is a homopolymer ofvinylidene fluoride or a vinylidene fluoride-tetrafluoroethylenecopolymer having a vinylidene fluoride contect of at least 50 mole %.12. The dust cover of claim 3, wherein said fluoropolymer is ahomopolymer of vinylidene fluoride or a vinylidenefluoride-tetrafluoroethylene copolymer having a vinylidene fluoridecontent of at least 50 mole %.
 13. The dust cover of claim 4, whereinsaid fluoropolymer is a homopolymer of vinylidene fluoride or avinylidene fluoride-tetrafluoroethlene copolymer having a vinylidenefluoride content of at least 50 mole %.
 14. The dust cover of claim 5,wherein said fluoropolymer is a homopolymer of vinylidene fluoride or avinylidene fluoride-tetrafluoroethylene copolymer having a vinylidenefluoride content of at least 50 mole %.
 15. The dust cover of claim 6,wherein said fluoropolymer is a homopolymer of vinylidene fluoride or avinylidene fluoride-tetrafluoroethylene copolymer having a vinylidenefluoride content of at least 50 mole %.
 16. The dust cover of claim 7,wherein said fluoropolymer is a homopolymer of vinylidene fluoride or avinylidene fluoride-tetrafluoroethylene copolymer having a vinylidenefluoride content of at least 50 mole %.
 17. The dust cover of claim 8,wherein said fluoropolymer is a homopolymer of vinylidene fluoride or avinylidene fluoride-tetrafluoroethylene copolymer having a vinylidenefluoride content of at least 50 mole %.
 18. The dust cover of claim 2,wherein said fluoropolymer is a tetrafluoroethylene(TFE)-hexafluoropropylene (HFP)-vinylidene fluoride (VdF) terpolymerhaving a TEF content of 35 to 65 mole % and an HFP: VdF molar ratio offrom 1:1 to 1:2.3.
 19. The dust cover of claim 3, wherein saidfluoropolymer is a tetrafluoroethylene (TFE)-hexafluoropropylene(HFP)-vinylidene fluoride (VdF) terpolymer having a TEF content of 35 to65 mole % and an HFP: VdF molar ratio of from 1:1 to 1:2.3.
 20. The dustcover of claim 4, wherein said fluoropolymer is a tetrafluoroethylene(TFE)-hexafluoropropylene (HFP)-vinylidene fluoride (VdF) terpolymerhaving a TEF content of 35 to 65 mole % and an HFP:VdF molar ratio offrom 1:1 to 1:2.3.
 21. The dust cover of claim 5, wherein saidfluoropolymer is a tetrafluoroethylene (TFE)-hexafluoropropylene(HFP)-vinylidene fluoride (VdF) terpolymer having a TEF content of 35 to65 mole % and an HFP:VdF molar ratio of from 1:1 to 1:2.3.
 22. The dustcover of claim 6, wherein said fluoropolymer is a tetrafluoroethylene(TFE)-hexafluoropropylene (HFP)-vinylidene fluoride (VdF) terpolymerhaving a TEF content of 35 to 65 mole % and an HFP:VdF molar ratio offrom 1:1 to 1:2.3.
 23. The dust cover of claim 7, wherein saidfluoropolymer is a tetrafluoroethylene (TFE)-hexafluoropropylene(HFP)-vinylidene fluoride (VdF) terpolymer having a TEF content of 35 to65 mole % and an HFP:VdF molar ratio of from 1:1 to 1:2.3.
 24. The dustcover of claim 7, wherein said fluoropolymer is a tetrafluoroethylene(TFE)-hexafluoropropylene (HFP)-vinylidene fluoride (VdF) terpolymerhaving a TFE content of 35 to 65 mole % and an HFP:VdF molar ratio offrom 1:1 to 1:2.3.